Java: Casting Object to a generic type

Question

In Java when casting from an Object to other types, why does the second line produce a warning related to the cast, but the first one doesn\'t?

void a(Object o)          


        

Answer 1

It's because the object won't really be checked for being a List at execution time due to type erasure. It'll really just be casting it to List. For example:

List strings = new ArrayList();
strings.add("x");
Object o = strings;

// Warning, but will succeeed at execution time
List integers = (List) o;
Integer i = integers.get(0); // Bang!

See Angelika Langer's Java Generics FAQ for more info, particularly the type erasure section.